Ratatouille is animated, so nothing here was filmed: these are the real places Pixar drew on for the story of Remy, a rat who dreams of being a chef. Start on the Left Bank at La Tour d'Argent, the restaurant that inspired Gusteau's, the restaurant in the film. Less than 10 minutes away is the Pont au Double, beside Notre-Dame: according to Sortir à Paris, this is where Remy and the young cook Linguini agree to work together.
The market street of Rue Mouffetard is about 15 minutes from La Tour d'Argent. Keep a second day for the Right Bank and the west: the shop window full of stuffed rats at Aurouze, near Châtelet, is about 10 minutes from the Pont des Arts seen in the opening images, while Place de la Concorde, Pont Alexandre III and the sewer museum at Pont de l'Alma are 10 to 15 minutes apart.
Le Train Bleu, the station restaurant at Gare de Lyon that helped inspire Gusteau's dining room, is on its own, so go by metro. At the Pont au Double, stand on the Quai de Montebello under the bridge: the stairs and the cathedral towers line up as in the film. The same bridge also appears in Charade.